Conventionally positioners, which are field devices that control the degrees of opening of regulator valves, are designed so as to operate with an electric current between 4 and 20 mA sent through a pair of electric wires from a higher-level system. For example, if a current of 4 mA is sent from the higher-level system, the opening of the regulator valve is set to 0%, and if a current of 20 mA is sent, then the opening of the regulator valve is set to 100%.
In this case, the electric current that is supplied from the higher-level system varies in the range of 4 mg through 20 mA, and thus the internal circuitry within the positioner produces its own operating power supply (the primary power supply) from an electric current of no more than 4 mA, which can always be secured as an electric current value that is supplied from the higher-level system. (See, for example, Japanese Unexamined Patent Application Publication 2004-151941.)
FIG. 5 is illustrates the critical components in a conventional positioner. This positioner 100 receives a supply of an electric current I through a pair of electric wires L1 and the L2 from the higher-level system 200 and produces a primary power supply from the electric current I that is supplied, and, on the other hand, also controls the degree of opening of a regulator valve, not shown, in accordance with the value of the supplied electric current I.
The positioner 100 is provided with a main circuit 3 that includes a CPU (calculation processing portion) 1 along with various types of functional circuit portions 2 (A/D converting devices, driving circuits for EPMs (electropneumatic converting devices), sensor circuits, digital circuits, and so forth), and a primary power supply generating circuit portion 4 that includes a zener diode D1. In this positioner 100, the primary power supply generating circuit portion 4 produces a constant voltage Vs from the supply electric current I from the higher-level system 200, and supplies that produced constant voltage Vs to the main circuit 3 as the primary power supply.
However, in the circuit structure illustrated in FIG. 5, even though the scope of the electric current of the supply electric current I wherein proper operation is possible is defined as a specification of the positioner 100, and even though there are no problems as long as the supplied electric current I ramps up quickly to the electric current range wherein proper operation is possible at the time of, for example, startup of the supply of power from the higher-level system 200 (referencing Curve I shown in FIG. 6), if the supplied electric current I changes slowly (referencing Curve II shown in FIG. 6), there is the risk that the main circuit 3 that includes the CPU 1 and the various types of functional circuit portions 2 will start up with the voltage generated by the primary power supply generating circuit portion 4 being inadequate, producing an erratic operating state, which may cause malfunctions such as a valve being opened unintentionally.
Note that Japanese Unexamined Patent Application Publication H3-212799 (the “JP '799 Application”, issued as Japanese Patent 2753592) shows a double-wire instrument that receives the supply of power (a voltage) through a two-wire transmission line, measures a physical quantity, such as a flow rate, and transmits an electric current signal in accordance with the measured value. In this double-wire instrument, drops in the terminal voltage are monitored, and if a drop in the terminal voltage is detected, the microprocessor is initialized and a warning is sent. However, even though there has been an attempt to solve the problem with the positioner, set forth above, through the application of the technology disclosed in this JP '799 Application, given the following facts, the problem cannot be solved easily.
[Fact 1]
The double-wire instrument described in the JP '799 Application is a voltage input-type instrument, but the positioner is an electric current input-type device, and thus the mode of operation is different.
[Fact 2]
While a case wherein a fault occurs, such as a drop in the power supply voltage from a state wherein the double-wire instrument is operating normally, can be handled by the technology described in the JP '799 Application, it is not possible to detect whether or not there have been proper operations.
The present invention was created in order to solve such problems, and an aspect thereof is to provide a field device able to prevent the occurrence of faults due to the calculation processing portion or various types of functional circuit portions operating in an unstable state.